Best known for his haunting, Oscar-nominated performance as Ennis Del Mar, one of the gay cowboys in 2005's"Brokeback Mountain,"Ledger was a massive young talent on the cusp of greatness when he died last week in New York. The native Australian, who is survived by his 2-year-old daughter, Matilda, had recently finished work on this summer's"Batman"sequel,"The Dark Knight,"in which he plays a villain, the Joker. Christopher Nolan, the film's director, shared these memories:

One night, as I'm standing on LaSalle Street in Chicago, trying to line up a shot for "The Dark Knight," a production assistant skateboards into my line of sight. Silently, I curse the moment that Heath first skated onto our set in full character makeup. I'd fretted about the reaction of Batman fans to a skateboarding Joker, but the actual result was a proliferation of skateboards among the younger crew members. If you'd asked those kids why they had chosen to bring their boards to work, they would have answered honestly that they didn't know. That's real charisma—as invisible and natural as gravity. That's what Heath had.

Heath was bursting with creativity. It was in his every gesture. He once told me that he liked to wait between jobs until he was creatively hungry. Until he needed it again. He brought that attitude to our set every day. There aren't many actors who can make you feel ashamed of how often you complain about doing the best job in the world. Heath was one of them.

One time he and another actor were shooting a complex scene. We had two days to shoot it, and at the end of the first day, they'd really found something and Heath was worried that he might not have it if we stopped. He wanted to carry on and finish. It's tough to ask the crew to work late when we all know there's plenty of time to finish the next day. But everyone seemed to understand that Heath had something special and that we had to capture it before it disappeared. Months later, I learned that as Heath left the set that night, he quietly thanked each crew member for working late. Quietly. Not trying to make a point, just grateful for the chance to create that they'd given him.

Those nights on the streets of Chicago were filled with stunts. These can be boring times for an actor, but Heath was fascinated, eagerly accepting our invitation to ride in the camera car as we chased vehicles through movie traffic—not just for the thrill ride, but to be a part of it. Of everything. He'd brought his laptop along in the car, and we had a high-speed screening of two of his works-in-progress: short films he'd made that were exciting and haunting. Their exuberance made me feel jaded and leaden. I've never felt as old as I did watching Heath explore his talents. That night I made him an offer—knowing he wouldn't take me up on it—that he should feel free to come by the set when he had a night off so he could see what we were up to.

When you get into the edit suite after shooting a movie, you feel a responsibility to an actor who has trusted you, and Heath gave us everything. As we started my cut, I would wonder about each take we chose, each trim we made. I would visualize the screening where we'd have to show him the finished film—sitting three or four rows behind him, watching the movements of his head for clues to what he was thinking about what we'd done with all that he'd given us. Now that screening will never be real. I see him every day in my edit suite. I study his face, his voice. And I miss him terribly.

Back on LaSalle Street, I turn to my assistant director and I tell him to clear the skateboarding kid out of my line of sight when I realize—it's Heath, woolly hat pulled low over his eyes, here on his night off to take me up on my offer. I can't help but smile.